The proposed research improves our understanding of the zoonotic transmission of human granulocytic ehrlichiosis. The etiologic agent of this newly emerging, sometimes fatal zoonotic disease is antigenically and genetically almost identical to a well described pathogen of horses (equine granulocytic ehrlichiosis) that occurs most frequently in areas corresponding to the natural range of the suspected tick vector, Ixodes pacificus. Midwestern and eastern HGE cases have been associated with exposure to the eastern tick Ixodes scapularis. Because L scapularis does not occur west of the Rocky Mountains, and because aspects of HGE and E. equi epidemiology appear to differ between California and east coast cases, it is necessary to explore the ecology of HGE in the specific context of disease "hot spots" in the California coast range. Such an exploration would consist of identifying naturally infected reservoir species, confirming the vector species, and identifying routes of zoonosis that serve to place humans at risk for disease. ThIs study will thoroughly describe the epidemiology and natural history of HGE, and will serve as a training grant allowing Dr. Foley to develop skills as an independent researcher. Dr. Foley is ideally suited for thin project, because she has previous training in internal medicine, wildlife biology, and disease ecology. This project, when complete, will provide us with information about the spatial and temporal distribution of HGE, and major zoonotic risk factors for humans acquiring this possibly fatal disease.